A COMPARISON OF (TH,PU)O-2 AND UO2 FUELS AS WASTE FORMS FOR DIRECT DISPOSAL

Issues relevant to the performance of irradiated (Th,Pu)O-2 as a waste form for geological disposal are briefly reviewed. Fuels of this type are among those being considered for burning plutonium from dismantle d nuclear weapons in power reactors, including Canada deuterium uraniu m (CANDU) systems. The high chemical stability and low aqueous solubil ity of thoria make this type of fuel attractive as a waste form. In co ntrast with UO2 fuel, the inertness of thoria to oxidation dominates m ost of the chemical issues of fuel disposal. The overall performance o f a thoria-based fuel waste form is likely to be determined by the ''i nstant'' release of the gap inventories of mobile fission products suc h I-129. This in turn will be controlled largely by the inas reactor p ower history and probably also by details of fuel fabrication. Limited experience with thoria-based fuels [chiefly (Th,U)Oz] indicates that, for given power and burnup levels, gas releases can be substantially, lower than with UO2 fuels. The gap and grain-boundary inventories of fission products are expected to be correspondingly low. A fabrication route involving molecular-level mixing (e.g., sol-gel process) would be preferable to powder blending, because microscopic heterogeneities in the fuel might adversely affect the retention of fission products. Pilot-scale irradiation, postirradiation examination, and leaching stu dies are required to support this preliminary assessment. Other issues that need to be addressed include impurity specifications (to minimiz e formation of long-lived activation products) and criticality and saf eguards issues that might influence the design of fuel-handling facili ties.